1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary cell having a positive electrode composed mainly of a positive electrode active material, a negative electrode, and a non-aqueous electrolyte.
2 Description of the Prior Art
In recent years, non-aqueous electrolyte cells have become the focus of considerable attention as a type of cell that can achieve high capacity. Non-aqueous electrolyte cells typically employ a lithium-containing complex oxide such as lithium cobalt oxide as a positive electrode material, and a material capable of reversibly absorbing and desorbing lithium ions such as a lithium-aluminum alloy, a carbon material, and the like as a negative electrode material.
It is known that the lithium cobalt oxide deteriorates as charge-discharge cycles are repeated. The degree of the deterioration is related to the crystallinity of the lithium cobalt oxide, and the deterioration of the structure caused by charge-discharge operations is more noticeably exhibited when the crystallinity of the lithium cobalt oxide is low. When the crystallinity of the lithium cobalt oxide is low, the lithium cobalt oxide tends to be easily decomposed at charge state and therefore the desorption of oxygen in the active material occurs more easily, which causes degradation in the thermal stability of the cell.
In view of the problem, it may be possible that the crystallite size of the lithium cobalt oxide is increased to improve the crystallinity of the lithium cobalt oxide. However, this technique has a problem such that merely increasing the crystallite size of the lithium cobalt oxide causes a decrease in the diffusion rate of lithium and thereby the degradation in the discharge characteristic.
Another technique for obviating the problem that has been proposed is such that a portion of cobalt in the lithium cobalt oxide in a cell is replaced by another element to improve the discharge characteristic. However, when a portion of the cobalt is replaced by another element, the crystal growth is hindered and consequently the crystallite size becomes small, which causes degradation in thermal stability at charge state.
For these reasons, it has not been feasible to construct a cell that can satisfy sufficient cycle life characteristic, thermal stability, and discharge characteristic, all of which are fundamental characteristics required for a cell.
In view of the foregoing and other problems of the prior art, it is an object of the present invention to provide a non-aqueous electrolyte secondary cell capable of increasing thermal stability of the positive electrode active material and of improving the discharge characteristic and charge-discharge cycle characteristic of the cell.
This and other objects are accomplished in accordance with the present invention by providing a non-aqueous electrolyte secondary cell comprising:
a positive electrode mainly composed of a positive electrode active material;
a negative electrode; and
a non-aqueous electrolyte;
wherein:
the positive electrode active material comprises a lithium-containing transition metal complex oxide with hexagonal structure represented by the general formula LiCo1-XMXO2, wherein M is at least one element selected from the group consisting of V, Cr, Fe, Mn, Ni, Al, and Ti, and
a crystallite size of the lithium-containing transition metal complex oxide with respect to a (110) direction is greater than 1000 xc3x85.
The lithium-containing transition metal complex oxide in which the crystallite size is more than 1000 xc3x85 has a high crystallinity. Therefore, the deterioration of the lithium-containing transition metal complex oxide caused by charge-discharge cycling is prevented, and the lithium-containing transition metal complex oxide is not easily decomposed during charge. Consequently, the desorption of oxygen in the active material is suppressed, and the thermal stability of the cell is thereby improved. In addition, because of the addition of the element M, the ionic conductivity of the positive electrode active material increases, which leads to an improvement in the discharge characteristic even when the crystallite size is large.
In addition, M in the general formula LiCo1-XMXO2 may be at least one element selected from the group consisting of Cr, Mn, Al, and Ti.
According to the above constitution of the invention, the ionic conductivity of the positive electrode active material is further increased, and thereby the discharge characteristic is further improved.
In addition, the value X in the general formula LiCo1-XMXO2 may be within the range of from 0.0001 to 0.005.
The reason why the value X is thus restricted is as follows. On one hand, if the value X is less than 0.0001, the advantageous effect caused by adding the element M cannot be sufficiently exhibited, and therefore the ionic conductivity of the positive electrode active material cannot be sufficiently increased and the discharge characteristic cannot be improved to a sufficient degree either. On the other hand, if the value X exceeds 0.005, the relative amount of cobalt reduces, which causes a decrease in the capacity of the positive electrode active material. In addition, the lithium-containing transition metal complex oxide may be made from an oxide of cobalt having a specific surface area of 1 m2/g or larger or a cobalt-containing complex oxide having a specific surface area of 1 m2/g or larger.
The oxide of cobalt or the cobalt-containing complex oxide having a large specific surface area (having a specific surface area of 1 m2/g or larger) has a high reactivity, and therefore, the resulting lithium-containing transition metal complex oxide, which is produced by mixing the oxide with a lithium source such as lithium carbonate and then calcining the mixture, exhibits a high crystallinity. In addition, since these oxides has a high reactivity, the degradation of the crystallinity is suppressed even when the element M is added.